You can check our list of weather overlays where it is described as follows: "Cloud base height is the altitude of the lowest cloud base above the ground, especially important for pilots. The model of terrain is simplified, so be cautious to use these values in the mountains. On large areas, the forecast model is not able to calculate the cloud base."

When talking about 'cloud bases', the clock icon indicates that the model of terrain is simplified. Can you indicate how that simplification works. I live in the mountains. Does it average valleys and tops of mountains? AGL doesn't mean much unless you know the height of the terrain.